Experimental thinning reduces inter-ovary competition among and within inflorescences

Resource availability often limits female reproductive success in plants, especially when multiple flowers within inflorescences compete simultaneously for limited resources. Here, I explore whether relaxation of inter-ovary competition (through experimental thinning of inflorescences and/or inflorescence buds) influences resource allocation and enhances reproductive output in Lupinus polyphyllus, a legume species with vertical inflorescences that typically show a decline in fruit and seed production from basal to distal flowers.

I implemented a gradient of reproductive thinning: (1) no removal (control); (2) removal of half of the currently open inflorescences; and (3) removal of both half of the open inflorescences and all inflorescence buds (i.e. future racemes). For three randomly selected remaining inflorescences per plant, I recorded the total number of fruits within three sections along the inflorescence (basal, middle and distal). For each fruit in each of these three inflorescences, I also counted the number of viable seeds, aborted seeds and unfertilized ovules.

The results strongly supported the inter-ovary competition hypothesis. Inflorescence removal increased fruit and viable seed production and reduced seed abortion, especially when both inflorescences and buds were removed. These effects occurred consistently across all positions within inflorescences, suggesting enhanced resource allocation even to distal flowers, which are often resource limited. Ovule production per flower was unaffected, indicating no over-compensation prior to fertilization. The number of fertilized ovules declined from basal to distal flowers, consistent with non-uniform pollen receipt, probably influenced by bumblebee foraging behaviour. However, fertilization ratios remained high (80-90 %) across positions and treatments.

These findings demonstrate that inflorescence thinning can effectively relax resource-based constraints within and among inflorescences, enhancing female success without altering pollination dynamics. This contributes to our understanding of how architectural plasticity and developmental constraints shape reproductive trade-offs in flowering plants.